Add FAQ entry to describe how users might include PIDs in data files

[squaregoldfish]

In the CF Conventions Workshop we decided that CF should not add attributes for dataset IDs (e.g. DOIs) and related items as part of their conventions. However, we should and an FAQ entry to address this issue, indicating that users should adopt additional conventions that do handle these cases (e.g. ACDD).

Since we had a volunteer (Mark Hedley) to provide support for ACDD usage (and they reference CF in their documentation) it is reasonable to explicitly mention that as a possible path for userst to take.

[JonathanGregory]

Dear Steve @squaregoldfish

I agree that this is a frequently asked question, and it would be useful to add it to the FAQ. Would you be willing to draft some text?

Best wishes

Jonathan